Antioxidant activity of extract of adzuki bean and its fractions

Phenolic compounds were extracted from adzuki bean using 80% (v/v) aqueous acetone. Crude extract was applied onto a Sephadex LH-20 column. Fraction I, consisting of low-molecular-weight phenolics, was eluted from the column using ethanol. Fraction II, consisting of tannins, was obtained using water/acetone (1:1; v/v) as the mobile phase. Phenolic compounds present in the crude extract and its fractions showed antioxidant and radical scavenging properties as revealed using a beta-carotene-linoleate model system, the total antioxidant activity (TAA) method, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and reducing power evaluation. Results of these assays showed highest values when tannins (fraction II) were tested. For example, the TAA of the tannin fraction was 4.17 mu mol Trolox/mg, whereas extract and fraction I showed 1.76 and 1.40 mu mol Trolox/mg, respectively. The content of total phenolics in fraction II was the highest (189 mg/g). The content of tannins in this fraction determined using the vanillin method and expressed as absorbance units at 500 nm/1 g was 213. There were 29 compounds (hydroxycinnamics, procyanidins, gallates, flavonols, dihydroflavonols, dihydrochalcones) identified in the crude extract using a high-performance liquid chromatography with photodiode array and mass spectrometry detectors (HPLC-PAD-MS) method. Catechin and epicatechin glucosides, procyanidin dimers, myricetin and protocatechuic acid were the dominant phenolics in the extract.